# How to calculate force exerted by ground

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1. Sep 4, 2016

1. The problem statement, all variables and given/known data

Force exerted by the ground on the rocket

Mass of the Rocket = 1500kg
Duration of landing impact = 0.04 s
Velocity at time of impact = 175 m/s
Distance travelled = 30km
Time taken to complete journey = 6 minutes
Acceleration due to gravity = 9.8 m/s^2

2. Relevant equations
Can't find the one im looking for

3. The attempt at a solution
I

Not going to lie i've attempted it but i dont know what formula to use and ive done all the questions before except one, please may someone tell me the equation I am supposed to use?

2. Sep 4, 2016

### TomHart

So the problem is to find the force exerted by the ground on the rocket? You are given 0.04 s as the duration of landing impact. And you are given the velocity at the start of the impact is 175 m/s. So you have three things that you know: initial velocity, final velocity and time. What can you find from knowing those three parameters?

3. Sep 4, 2016

### haruspex

It is unclear what is being asked for. Is it the average force during impact, or the average force over the duration of flight plus impact?
Please post the original question word for word.

4. Sep 5, 2016

Calculate the force exerted by the ground on the rocket (use correct units)

Mass of the Rocket = 1500kg
Duration of landing impact = 0.04 s
Velocity at time of impact = 175 m/s
Distance travelled = 30km
Time taken to complete journey = 6 minutes
Acceleration due to gravity = 9.8 m/s2

I got:
Final velocity = 175 m/s
Initial velocity = u=v-at → u=175-(9.8*360) = -3353

Force exerted by the ground on the rocket (N)

F = mass*acceleration
Acceleration = change in velocity/time = Final Vel.-Initial Vel./0.04 = 175-(-3353)/0.04=84000m/s2
F = 1500*84000 = 126000000 N

5. Sep 5, 2016

### haruspex

Not sure what you think you are calculating there. You seem to be saying that at the start of the 6 minutes the rocket was going vertically upward with speed u, that for the remainder of the 6 minutes it was in free fall, so that at the end of the 6 minutes it was falling at 175 m/s. None of that sounds right to me. For a start, to finish at ground level it would have had to start some kilometres below ground.

You description of the question in post #4 still does not read like a complete statement. Is there nothing before that? I would expect it to start with some text like "A rocket is fired from the ground..."
And I still cannot tell whether it is asking for the average force during impact or the average force over the whole trajectory.

6. Sep 5, 2016

### CWatters

AdeptDz... The equations of motion you are using only work for constant acceleration. In this problem the acceleration of the rocket in flight is not the same as when it hits the ground.

So you have to break the problem into different phases and apply the equations to each phase separately.

This means that the "Initial velocity" you need to plug into the equation is the initial velocity of that PHASE which is not necessarily the initial velocity at the start of the problem.

This question asks about the impact force with the ground so the relevant initial velocity is the velocity with which it hits the ground not the launch velocity.